CORRELATION BETWEEN STRUCTURE AND ELECTRO N-TRANSFER MODALITY OF MODEL ORGANOMETAL COMPOUNDS FOR FLAVINS WITH BUTTERFLY CONFORMATION

The crystal structure analysis of a highly substituted 1,4-dihydropyra zine is described by example of the 4-bis(tert-butyldimethylsilyl)-2,3 ,5,6-tetramethyl derivative 2. In the solid state, the heterocycle exh ibits pronounced deviation from planarity with a dihedral angle of 140 .8-degrees along the N-N' axis of the butterfly-shaped 8 pi-electron r ing. The alkyl groups of the Si(tert-butyl)Me2 substituents are arrang ed to minimize steric repulsion. Despite the strongly reduced pi-elect ron conjugation caused by non-planarity, compound 2 is oxidized revers ibly to an ESR detectable radical cation 2+. whereas the second oxidat ion to the fully aromatic dication 2(2+) is electrochemically irrevers ible. The strongly diminished difference between the anodic peak poten tials for the two oxidation steps correlates with the non-planar confo rmation as is shown by comparison with similar 1,4-bis(tri-alkyl- or d ialkyl-silyl)-1,4-dihydro-1,4-diazines. The results support the concep t of structural-controlled one electron vs. two electron transfer of f lavin prosthetic groups in flavoenzymes.